Using time domain reflectometry for monitoring mineralization of nitrogen from soil organic matter

The mineralization of nitrogen from soil organic matter is important when o ne tries to optimize nitrogen fertilization and assess risks of N losses to the environment, but its measurement is laborious and expensive. We have e xplored the possibilities for monitoring N mineralization directly using ti me domain reflectometry (TDR). Net N and S mineralization were monitored ov er a 101-day period in two layers (0-30 and 30-60 cm) of a loamy sand soil during aerobic incubation in a laboratory experiment. At the same time elec trical conductivity of the bulk soil, sigma(a), was measured by TDR. A seri es of calibration measurements with different amounts of KNO3 at different soil moisture contents was made with the topsoil to calculate the electrica l conductivity, sigma(w), of the soil solution from sigma(a) and theta. The actual sigma(w) was determined from the conductivity of 1:2 soil:water ext racts (sigma(1:2)) with a mass balance approach using measured NO3- concent rations, after correction for ions present prior to the addition of KNO3. T he average N mineralization rate in the topsoil was small (0.12 mg N kg(-1) day(-1)), and, as expected, very small in the subsoil (0.023 mg N kg(-1) d ay(-1)). In the top layer NO3- concentrations calculated from sigma(a) dete rmined by TDR slightly underestimated measured concentrations in the first 4 weeks, and in the second half of the incubation there was a significant o verestimation of measured NO3-. Using the sum of both measured NO3- and SO4 2- reduced the overestimation. In the subsoil calculated NO3- concentration s strongly and consistently overestimated measured concentrations, although both followed the same trend. As S mineralization in the subsoil was very small, and initial SO42- concentrations were largely taken into account in the calibration relations, SO42- concentrations could not explain the overe stimation. The very small NO3- and SO42- concentrations in the B layer, at the lower limit of the concentrations used in the calibrations, are a possi ble explanation for the discrepancies. A separate calibration for the subso il could also be required to improve estimates of NO3- concentrations.